The present invention is a permanent magnet generator designed to be coupled with a power source such as a steam turbine. It is ideally suited for application in nuclear power plants. Natural disasters, for example the May 2011 disaster following the tsunami at the Tokyo Electric Fukushima Nuclear Power Plant, evidence a flaw in the design of emergency reactor cooling water systems with potentially devastating consequences. Lessons learned from past disasters include the irrefutable conclusion that the electrical power primary and backup systems feeding the reactor core cooling injection pump drive steam turbines are subject to failure by natural disaster leading to a potential reactor core melt down, danger to and loss of human life, and long term irreversible environmental damages.
In order to lessen the probability of disaster and its associated consequences, the steam turbine controls for the Safety Related (as defined by the United States Congressional Federal Register 10 CFR 50 Part B) steam turbines need to have no reliance upon the plant power feeds. The Safety Related steam turbine control components of the speed governor and electric valve actuator for operating the turbine governor valve need to be fully self-powered by a source of the mechanical energy of the controlled turbine, thereby independent of the external power sources or plant-run power feeds that are commonly subject to failure in a natural disaster.
A complication of Safety Related turbine speed control is the “open governor valve” start position to place the turbine in operation. To be prepared for emergency pumping tasks, the Safety Related turbine governor valve actuator has the governor valves initially open in a fail-safe position under spring load. When the steam pressure is applied to the turbine (by an external valve), the turbine immediately begins acceleration from rest. In common applications, there is no acceleration control. Some nuclear plants have lessened acceleration by implementing small bypass steam lines admitting less steam flow and resulting in more gradual turbine rotor acceleration, but all operate on a similar starting logic. Any proposed turbine speed control system has the task of becoming functional at a low turbine shaft speed, at or near 500 revolutions per minute (RPM), and responding to limit the initial speed surge. Failure to respond by closing the turbine governor valve to the speed throttling position quickly results in excess acceleration and turbine over-speed trip, or safety shut-down of the turbine. Original equipment turbine speed control systems from the previous century were plagued with poor responding hydraulic control systems which often could not retard the acceleration quick enough due to susceptibility to operating oil contamination, air in the hydraulic oil and friction from long term inactivity.
Conventional permanent magnet generators can be coupled to turbine shafts to produce electrical power, but cannot provide electrical power over the required wide speed range, typically 500 RPM to 5,000 RPM due to basic electromagnetic properties. If a permanent magnetic generator coupled to a Safety Related turbine is designed for proper coil output voltage at 500 RPM for a control system power feed, the coil output voltage will increase proportional to further turbine speed increase. This results in a ten-fold over-voltage output at 5,000 RPM which exceeds potentials and which will likely destroy electrical components in the rectification and shunt voltage regulation or limitation circuits.
Newer generation Safety Related turbine speed control designs have implemented the use of electric actuators utilizing electric motors and roller screw engagement devices to position governor valves. Although the electric actuator represents a vast improvement over the previous hydraulic systems in reliability and reduced maintenance requirements, until this invention there was no means to power the electric actuator and connecting servo drive other than with plant AC or DC busses which are typically the items of failure in a natural disaster, including the tsunami at Fukushima.
Previous work has established some degree, but not total turbine self-powering. For example, U.S. Pat. No. 5,789,822 to Calistrat et al. utilizes the low power generation of magnetic speed probes to self-power the electronic governor, but does not address the much greater electrical power demand of operating the governor steam valve and therefore must use a non-electric, hydraulic-positioned governor valve with accompanying high failure potential and complexity.
Other work has centered on designs of permanent magnet generators for general applications which either have no voltage regulating capability or use complex electrical apparatus to compensate for limited variable speed operation. Due to the required radiation survival criteria for Safety Related turbine applications, complex electrical apparatus is not feasible, nor reliable, and the extreme range of speed of operation of a Safety Related turbine, again typically 500 RPM to 5,000 RPM, at a 1:10 ratio, is beyond the compensating ranges of the prior art. Any suitable device must withstand an environment having radiation levels on the magnitude of 105 rads.
Further art has uniformly centered on devices and configurations to improve the generation efficiency of permanent magnet generators, but none is like the subject invention which utilizes a decrease in generation efficiency to simplify regulation and make the power feed system more robust with fewer failure potential components.
The physical components of permanent magnet generators in basic form consist of sets of permanent magnets and wire-wound coils in proximity under a relative velocity. A key property of permanent magnet generators is the magnet-to-coil proximity, also known as the “air gap”. The magnetic flux density of the magnets decreases proportionately with the magnitude of the air gap. The generated voltage across a coil is proportional to the flux density at a given relative velocity, and increases proportionately with relative velocity.
The generated voltage can be expressed with the following formula:V=NdI′/dt where                V=voltage generated at each stator coil        I′=instantaneous value of the magnetic flux cutting the stator coil under magnetic rotation        I=peak magnetic flux density at near-zero air gap        s=air gap distance        N=number of stator coil turns and        I′=I/s        
Therefore a voltage compensation of increasing velocity can be accomplished by simultaneous increasing the air gap at the cost of decreasing generator efficiency. Since efficiency is of minor importance in light of Safety Related turbine operation, and the overall device mechanical load on the turbine is small, sacrificing efficiency for robust power generation is a good trade off.